Four wheel steering system for vehicle handling improvement: A robust model reference control using the sliding mode

In this paper a MIMO model reference control scheme incorporating the sliding mode control theory for a vehicle four wheel steering system is proposed and evaluated for a class of continuous-time non-linear dynamics with uncertainties. By the Lyapunov direct method, the algorithm is proven to be globally stable, with tracking errors converging to the neighbourhood of zero. The sliding mode four wheel steering (SM4WS) not only improves the directional stability and responsiveness but also provides good disturbance rejection for unexpected side wind. The SM4WS results in a faster vehicle response than conventional two wheel steering (2WS) without suspension and tyre modification. The linear three degree-of-freedom vehicle handling model is used to investigate vehicle handling performances. In simulation of the J-turn, the yaw rate overshoot reduction of a typical mid-size cap improved by 30 per cent compared to a 2WS case. Although the lateral deviation of SM4WS is almost the same as that of 2WS the yaw rate reduction was approximately 20 per cent of that of a 2WS system. The simulation of the J-turn manoeuvre shows that the proposed scheme gives faster yaw rate response and smaller slide-slip angle than the 2WS case. When the rear tyre pressure is lower than normal, the car has less understeer and is worse in directional responsiveness, while SM4WS is insensitive to such parameter variations because of the robustness characteristics of the sliding mode control.